1. Field of the Invention
The present invention pertains to methods and apparatus involving smooth electrodes and thin film ferroelectrics for use in integrated circuits. More particularly, a bottom electrode is DC-sputter deposited in a special carrier gas mixture to improve the memory retention characteristics of a ferroelectric capacitor.
2. Statement of the Problem
Thin film ferroelectric materials are used in a variety of nonvolatile random access memory devices. For example, U.S. Pat. No. 5,600,587 issued to Koike teaches a ferroelectric nonvolatile random access memory using memory cells consisting of a ferroelectric capacitor and a switching transistor. U.S. Pat. No. 5,495,438 issued to Omura teaches a ferroelectric memory that is formed of ferroelectric capacitors connected in parallel. The capacitors have ferroelectric materials of different coercive field values and, consequently, can use or store multi-value data. U.S. Pat. No. 5,592,409;issued to Nishimura et al. teaches a nonvolatile memory including a ferroelectric layer that is polarized by the impressed voltage between two gates. The polarization or memory storage state is read as a high or low current flow across the ferroelectric layer, which permits nondestructive readout. U.S. Pat. No. 5,539,279 issued to Takeuchi et al. teaches a high speed one transistor one capacitor ferroelectric memory that switches between two modes of operation including a dynamic random access memory (xe2x80x9cDRAMxe2x80x9d) mode and a ferroelectric random access memory (xe2x80x9cFERAMxe2x80x9d) mode.
Ferroelectric memories are nonvolatile because the ferroelectric materials polarize in the presence of an applied field and retain the polarization even after the applied field is removed. FIG. 1 depicts an ideal polarization hysteresis curve 100 for ferroelectric thin films. Side 102 of curve 100 is produced by measuring the charge on a ferroelectric capacitor while changing the applied field from a positive value to a negative value. Side 104 of curve 100 is produced by measuring the charge on the ferroelectric capacitor while changing the applied field E from a negative value to a positive value. The points xe2x88x92Ec and Ec are conventionally referred to as the coercive field that is required to bring polarization P to zero. Similarly, the remanent polarization Pr or xe2x88x92Pr is the polarization in the ferroelectric material at a zero field value. The Pr and xe2x88x92Pr values ideally have the same magnitude, but the values are most often different in practice. Thus, polarization measured as 2Pr is calculated by adding the absolute values of the actual Pr and xe2x88x92Pr values even though these values may differ in magnitude. The spontaneous polarization values Ps and xe2x88x92Ps are measured by extrapolating a linear distal end of the hysteresis loop, e.g., end 106, to intersect the polarization axis. In an ideal ferroelectric, Ps equals Pr, but these values differ in actual ferroelectrics due to linear dielectric and nonlinear ferroelectric behavior. A large, boxy, substantially rectangular central region 108 shows suitability for use as a memory by its wide separation between curves 102 and 104 with respect to both coercive field and polarization.
Ferroelectric memories are fast, dense, and nonvolatile. Even so, ferroelectric memories do not enjoy widespread commercial use, in part, because the polarization of a thin film ferroelectric material degrades with repeated use. Actual thin film ferroelectrics do not perform as ideal ferroelectrics. Deviation from the ideal behavior of FIG. 1 is observed as ferroelectric imprint and fatigue. These deviations are so common and severe that it is nearly impossible to find thin film ferroelectrics which meet commercial requirements. The best materials for integrated ferroelectric devices are switched using a coercive field that can be obtained from conventional integrated circuit operating voltages, i.e., three to five volts (xe2x80x9cVxe2x80x9d). The materials should have a very high polarization, e.g., one exceeding twelve to fifteen micro coulombs per square centimeter (xe2x80x9cxcexcC/cm2xe2x80x9d) determined as 2Pr, to permit the construction of memories having sufficient densities. Polarization fatigue should be very low or nonexistent over hundreds of millions of switching cycles. Furthermore, the ferroelectric material should not imprint, i.e., the hysteresis curve should not shift to favor a positive or negative coercive field.
FIG. 2 depicts the effects of environmental stress on hysteresis curve 100. Curve 200 shows the effect of fatigue on curve 100. Fatigue reduces the separation between curves 102 and 104 defining central region 108. Central region 108 progressively becomes smaller and smaller with additional fatigue. This change in separation is primarily due to the creation of point charge defects arising in the ferroelectric material as a consequence of polarization switching together with the associated screening effect of the charge defects on the applied field. Thus, fatigue causes the ferroelectric material to wear out over time due to repeated polarization switching.
U.S. Pat. No. 5,519,234 issued to Araujo et al. teaches that the fatigue problem of curve 200 is substantially overcome by the use of layered superlattice materials, such as the xe2x80x9clayered perovskite-likexe2x80x9d materials described in Smolenskii et al. xe2x80x9cFerroelectrics and Related Materials,xe2x80x9d Gordon and Breach (1984). The use of thin film layered superlattice materials in integrated circuits was unknown prior to Dr. Araujo""s work. The layered superlattice materials are reported to provide a thin film ferroelectric material wherein the polarization state may be switched up to at least 109 times with less than thirty percent fatigue. This level of fatigue endurance provides a significant advance in the art because it is at least about three orders of magnitude better than the fatigue endurance of other ferroelectrics, e.g., lead zirconium titanate (xe2x80x9cPZTxe2x80x9d) or lead lanthanum zirconium titanate (xe2x80x9cPLZTxe2x80x9d). Prior layered superlattice material work has been done primarily with the use of a Pt/Ti bottom electrode and layered superlattice material films on the order of 1800 xc3x85 thick. The titanium is used as an adhesion layer to prevent peeling of the electrode from the substrate.
According to section 15.3 of the Smolenskii book, the layered perovskite-like materials or layered superlattice materials can be classified under three general types:
(A) compounds having the formula Amxe2x88x921Bi2MmO3m+3, where A=Bi3+, Ba2+, Sr2+, Ca2+, Pb2+, K+, Na+ and other ions of comparable size, and M=Ti4+, Nb5+, Ta5+, Mo6+, W6+, Fe3+ and other ions that occupy oxygen octahedra;
(B) compounds having the formula Am+1MmO3m+1, including compounds such as strontium titanates Sr2TiO4, Sr3Ti2O7 and Sr4Ti3O10; and
(C) compounds having the formula AmMmO3m+2, including compounds such as Sr2Nb2O7, La2Ti2O7, Sr5TiNb4O17, and Sr6Ti2Nb4O20.
Smolenskii observed that the perovskite-like layers may have different thicknesses, depending on the value of m, and that the perovskite AMO3 is in principal the limiting example of any type of layered perovskite-like structure with m=infinity. Smolenskii also noted that if the layer with minimum thickness (m=1) is denoted by P and the bismuth-oxygen layer is denoted by B, then the type I compounds may be described as . . . BPmBPm. . . . Smolenskii further noted that if m is a fractional number then the lattice contains perovskite-like layers of various thicknesses, and that all the known type I compounds are ferroelectrics.
Despite the tremendous improvements in low fatigue ferroelectrics attributable to layered superlattice materials, there remains an imprint problem that is typified by curve 202 of FIG. 2. Curve 202 shows that environmental stresses can imprint curve 100 by shifting it to the right or left. This imprinting occurs when the ferroelectric material is subjected to repetitive unidirectional voltage pulses. Some imprinting also occurs as a result of normal hysteresis switching, especially at a high temperature. The ferroelectric material retains a residual polarization or bias that shifts sides 102 and 104 in a positive or negative direction with respect to the applied field. Thus, curve 202 has been shifted in a positive direction 204 by repeated negative pulsing of a ferroelectric capacitor. A shift in the opposite direction could also occur due to repetitive pulsing by opposite voltage. This type of pulsing represents what happens to the ferroelectric materials as a consequence of repeated unidirectional voltage cycling, such as the sense operations in FERAMs. Imprint can be so severe that the ferroelectric material can no longer retain a polarization state corresponding to a logical 1 or 0 value, i.e., imprint degradation eventually makes the ferroelectric unsuitable for use in a memory.
U.S. Pat. No. 5,592,410 issued to Verhaeghe refers to the ferroelectric imprint phenomenon as xe2x80x98compensation.xe2x80x99 The ""410 patent teaches that imprint can be reversed by pulsing voltage during the write cycle to return the hysteresis loop towards the unimprinted position of curve 100, as compared to curve 202. Thus, the imprint problem is reversed by special write operations in which the pulsed voltage is opposite the switching voltage. Despite the teaching of Verhaeghe ""410, the reverse voltage pulsing does not address the entire problem because the imprint phenomenon is a partially irreversible one. The observed imprinting reflects corresponding changes in microstructure of the ferroelectric crystal, e.g., the creation of point charge defects with associated trapping of polarized crystal domains. Many of these microstructural changes are not reversible.
FIG. 3 depicts the deleterious effects of fatigue and imprinting on ferroelectric memory read/write control operations. Memory control logic circuits require a minimum polarization separation window, i.e., a programming window, which is represented by region 300. Region 300 must be large enough to produce a sufficient read-out charge for memory operations, e.g., for the operation of memory sense amplifier circuits. An initial 2Pr separation window 302 declines over the lifetime of the ferroelectric memory device along tracks 304 and 306 until, after about ten years or so of constant normal use, the separation between tracks 304 and 306 is too small for conducting memory operations. This lifetime of normal use follows stress time line 308. Curve 310 is a polarization hysteresis curve from the same material that produced curve 100, but is measured on decline at a point in time along tracks 304 and 306. The remanent polarization values Rms and Rmn correspond to +Pr and xe2x88x92Pr for the fatigued and imprinted material. Rms and Rmn are defined as remanent polarization at zero field in the fatigued and imprinted hysteresis curve 310. Arrow 312 shows a quantity of positive polarization retention loss, which is primarily due to fatigue. Arrow 314 shows a quantity of negative polarization retention loss, which is primarily caused by imprint shifting of curve 312 relative to curve 100. Arrow 316 shows a quantity of voltage center shifting of curve 312 relative to curve 100. This voltage center shifting indicates imprintation of the ferroelectric material.
Not all prior research efforts have focused upon the development of new ferroelectric materials to overcome the fatigue and imprint problems. Nakamura, xe2x80x9cPreparation of Pb(Zr, Ti)O3 Thin Films on Ir and IrO2 Electrodesxe2x80x9d 33 Jpn. J. Appl. Phys. 5207-5210 (September 1994), teaches the use of RF magnetron reactive sputtering to produce Pt, Ir and IrO2 electrodes. The substrate temperature was held at 450xc2x0 C. while the RF sputtering occurred,, and the films were subjected to a post-deposition anneal of 400xc2x0 C. A PZT thin film was deposited atop the RF-sputter deposited bottom electrodes. The polarization (xe2x80x9cPrxe2x80x9d) of PZT on a conventional Pt/Ti electrode decreased by 50% after 108 cycles. In comparison, a device including PZT between IrO2 top and bottom electrodes fatigued only 5% after 108 cycles. The article hypothesizes that the improvement in fatigue endurance was due to incompletely oxidized IrO2, which partially reacted with the PZT at the electrode-ferroelectric boundary.
Oxygen carrier gasses have been used in RF-magnetron reactive sputtering to prevent accelerated sputtering gases from generating point charge defects by striking a dielectric thin film of barium strontium titanate. Joo et al., xe2x80x9cImprovement of leakage currents of Pt/ (Ba,Sr)TiO3/Pt capacitorsxe2x80x9d, 70 Appl. Phys. Lett. 3053-3055 (June 1997) shows RF-magnetron reactive sputtering to deposit platinum top electrodes over thin film barium strontium titanate dielectric material. RF-magnetron deposition was performed using a mixed Ar/O2 carrier gas. Oxygen ions in the carrier gas compensated oxygen vacancies in the barium strontium titanate dielectric to provide a significant reduction in leakage current. The RF-sputter deposited platinum had a columnar structure, which was believed to facilitate the transport of oxygen ions across the top platinum electrode. The use of oxygen carrier gas Ar/O2 (35/15) for twenty seconds resulted in the deposition of a 5 nm thick platinum film, while the use of Ar gas alone for forty seconds resulted in the deposition of a 95 nm thick platinum film. Thus, the article determined that it was sufficient to reduce leakage current by introducing oxygen gas only at an initial stage of sputtering of the top electrode. The deposition rate could, accordingly, be enhanced in subsequent stages through the use of pure Ar carrier gas.
There remains a need to provide a bottom electrode structure for thin film ferroelectric layered superlattice material capacitors that improves the fatigue endurance of the layered superlattice materials and makes the layered superlattice materials substantially free of imprint. Furthermore, there is a need to improve sputtering processes by increasing the deposition rates of sputtered metals when a reactive carrier gas mixture is used in the sputtering chamber.
It has been discovered that the imprint phenomenon represented as curve 202 in FIG. 2 is affected by surface irregularities on the ferroelectric film and defects in the ferroelectric film, e.g., those corresponding to hillocks on the bottom electrode in a thin film ferroelectric capacitor device or similar surface irregularities on the top of the ferroelectric film and clusters or porosity inclusions in the ferroelectric film. In particular, the prior art Pt/Ti bottom electrodes form sharp hillocks that are especially prone to increase the amount of imprinting and the prior art spun-on ferroelectric films include defects that are prone to degrade the fatigue endurance and memory retention. Thus, ferroelectric capacitors having electrodes with sharp irregularities offer inferior electronic performance in integrated memories. Furthermore, it has been discovered that the use of oxygen carrier gas while sputtering top electrodes can improve the fatigue endurance, polarization, memory retention, and imprint characteristics of thin film layered superlattice materials while at the same time yielding an essentially smooth top electrode.
The present invention overcomes the problems outlined above by providing a DC-magnetron reactive sputtering process that utilizes a reactive carrier gas mixture to yield electrodes that are essentially smooth or hillock-free. The smooth electrodes are used in combination with ferroelectrics, especially the layered superlattice materials. Ultra thin films of layered superlattice materials less than about 500 xc3x85 or 800 xc3x85 thick offer significant and surprising advantages in ferroelectric performance that have not previously been suspected.
The smooth electrodes are produced according to a novel DC sputtering process. A carrier gas mixture for use in the DC-sputter deposition includes a mixture of a noble gas and a reactive gas species for the sputtering of conductive metals and conductive metal oxides. The ferroelectric materials may be specially processed using liquid source misted chemical deposition (xe2x80x9cLSMCDxe2x80x9d) and rapid thermal processing (xe2x80x9cRTPxe2x80x9d) after deposition of the bottom electrode to present a similarly smooth surface for receipt of a top electrode on the ferroelectric layer. The LSMCD is the deposition technique to use a single stoichiometrically correct liquid precursor which has precisely controlled amounts of strontium-, bismuth-, tantalum-, and niobium-precursors to form strontium bismuth tantalum niobate film. After converting the liquid precursor into an aerosol, the atomize aerosol is injected, along with an inert carrier gas, into a vacuum chamber, and deposited evenly over a rotating substrate. The RTP is accomplished by conventional means using a halogen lamp or other high energy radiative thermal transfer device. The top electrode is also DC-magnetron-sputter deposited using a carrier gas mixture including a noble gas and a reactive gas species.
Reactive ionic species produced by the glow discharge of a DC-magnetron are available to compensate point charge defects that are formed by the impact of accelerated ions upon the substrate. The reactive gas species of the carrier gas mixture are preferably a gaseous species of a reagent that reacts to yield a preexisting material on the substrate or a material that will subsequently be deposited on the substrate. Alternatively, the reactive gas can be any gas that reacts to compensate lattice defects. For example, the charge reactive gas species are oxygen where the electrode is sputtered over a metal oxide, and the oxygen compensates oxygen defects. Similarly, the charge compensation portion is nitrogen where the electrode is sputtered over a nitride, or nitrogen may be used in an attempt to overcompensate oxygen defects in a metal oxide.
Where the DC-sputtered electrodes are used in combination with layered superlattice materials, the layered superlattice materials resist fatigue well and their conformity to the smooth bottom electrode improves their imprint performance in integrated ferroelectric memories, such as FERAMs. A corresponding reduction in point charge defects in the layered superlattice materials also improves the fatigue endurance and resistance to fatigue.
Smooth electrodes advantageously permit the use of increasingly thinner films of layered superlattice materials without shorting of the ferroelectric capacitors. The thin films show a surprising improvement in their memory retention windows because memory retention windows in the thinner materials can have a greater magnitude than exists in comparable thicker materials. One would expect just the opposite effect because a greater number of oriented ferroelectric domains in the thicker materials should provide a greater cumulative polarization effect, but this greater cumulative polarization effect is not observed in practice. Thus, the use of smooth electrodes and thin films permits the construction of much better ferroelectric memories.
A preferred thin film ferroelectric capacitor according to the present invention includes a bottom electrode having a first smooth surface, a ferroelectric thin film layered superlattice material without any clusters or porosity inclusions, and a top electrode having a second smooth surface. The most preferred layered superlattice materials are strontium bismuth tantalate and strontium bismuth niobium tantalate. The ferroelectric thin film layered superlattice material contacts the smooth surfaces of the electrodes and has a thickness ranging from 300 xc3x85 to 2500 xc3x85. A smooth surface on one of the electrodes is hereby defined as one in which all surface irregularity features protruding towards the thin film ferroelectric layered superlattice material protrude a distance less than twenty percent of the thickness in the ferroelectric thin film layered superlattice material thickness. It is also preferred that substantially all of the surface irregularities on the smooth electrode are rounded and essentially free of acute angles. Another way of defining a smooth surface is that the surface is smoother, i.e., having surface irregularities that are less sharp, less tall, and less numerous, than the surface irregularities of a comparable 2000 xc3x85/200 xc3x85 thick Pt/Ti stacked electrode deposited on silicon which has been annealed while exposed to oxygen at 700xc2x0 C. to 800xc2x0 C. for one hour.
Ferroelectric thin film layered superlattice materials for use in the invention typically have thicknesses ranging from 300 xc3x85 to 2500 xc3x85. Thicknesses above this range are also useful, though they are seldom needed. A more preferred range of layered superlattice material thickness is from 300 xc3x85 to 1100 xc3x85. This range is even more preferably from 400 xc3x85 to 1000 xc3x85, and is most preferably from 500 xc3x85 to 800 xc3x85. The prior art does not show layered superlattice materials having these small thicknesses, which are less than about 1300 xc3x85.
Ferroelectric capacitors of the invention demonstrate superior electrical performance. For example, select ferroelectric thin film layered superlattice materials are capable of providing a 1.5 V polarization or charge separation window of at least 7 xcexcC/cm2 after being stored for a hundred hours at 75xc2x0 C. These 75xc2x0 C. storage is very severe, as compared to normal integrated circuit operating temperature and, consequently, tend to accelerate retention. The 7 xcexcC/cm2 separation window is sufficient for proper interaction with conventional integrated memory control logic circuits. The separation window increases as film thickness decreases down to about 300 xc3x85. Layered superlattice material films thinner than about 300 xc3x85 crystallize differently and show porosity along grain or domain boundaries, which makes them unsuitable for use in ferroelectric capacitors.
Another aspect of superior electronic performance in the ferroelectric thin film layered superlattice materials according to the invention is superior resistance to imprintation. Select ferroelectric thin film layered superlattice materials demonstrate a hysteresis shift of less than 0.0163 V corresponding to the 3 V polarization separation window after 1010 cycles of 6 V square wave fatigue endurance switching, as described above.
Yet another aspect of superior electronic performance is the development of ultra thin ferroelectric layered superlattice material films that are essentially fatigue free. The use of smooth electrodes permits the use of ferroelectric thin films having less than about 2% of 2Pr degradation after being switched 1010 cycles using a 1 V triangular wave at 10,000 Hz. This exceptional ferroelectric performance comes from ultra thin films, e.g., those ranging from 300 xc3x85 to 1100 xc3x85 in thickness.
The smooth electrode structures can be produced through use of a DC glow discharge. In a preferred embodiment, the bottom electrode includes a platinum layer. This platinum layer is preferably deposited on an iridium layer. Other preferred bottom electrode structures produced using the DC glow discharge include a platinum layer deposited on an iridium oxide layer, a platinum layer deposited on a titanium nitride layer, a platinum layer deposited on a titanium oxide layer, a platinum layer deposited on a tantalum nitride layer, a platinum layer deposited on a tantalum oxide layer, a platinum layer deposited on a tungsten silicide layer, and a platinum layer deposited on a tungsten silicon nitride layer.
In other preferred embodiments, the platinum may be substituted by ruthenium in each of the above preferred embodiments to provide Ru, Ru/Ir, Ru/IrO2, Ru/WSi, or Ru/WSiN electrodes.
In yet other preferred embodiments, the platinum may be substituted by iridium to provide Ir, Ir/IrO2, Ir/WSi, or Ir/WSiN electrodes.
The process of making the ferroelectric capacitors includes careful control of thermal process conditions. A smooth bottom electrode is formed wherein substantially all surface irregularity features on a bottom electrode are rounded and essentially free of acute angles. This smoothness derives from a proper selection of electrode materials and anneal temperatures. For example, the need for smoothness requires the anneal to be performed at a temperature ranging from 180xc2x0 C. to 500xc2x0 C., and this temperature preferably does not exceed 450xc2x0 C.
Ferroelectric capacitors for use in FeRAMs and the like are made using liquid precursors. A liquid precursor is deposited on the bottom electrode to provide a precursor film by conventional spin-on, more preferably by LSMCD. The precursor film is contains a plurality of metals that are capable of yielding a ferroelectric layered superlattice material upon drying and annealing of the precursor film. Drying of the precursor film is done at a temperature less than 400xc2x0 C. to provide a dried precursor residue. The dried precursor residue is soft baked using rapid thermal processing (xe2x80x9cRTPxe2x80x9d) at an RTP temperature ranging from 525xc2x0 C. to 675xc2x0 C. for a period of time ranging from thirty seconds to five minutes. The RTP temperature more preferably ranges from 625xc2x0 C. to 650xc2x0 C., and is most preferably 650xc2x0 C., which is the highest temperature that consistently produces a smooth upper surface on the resultant soft baked precursor residue. The soft baked precursor residue is annealed in a diffusion furnace under oxygen at an anneal temperature ranging from 450xc2x0 C. to 650xc2x0 C. for a period of time ranging from thirty minutes to five hours. The anneal temperature more preferably ranges from 500xc2x0 C. to 560xc2x0 C., and is most preferably 525xc2x0 C., which is just barely sufficient to crystallize the ferroelectric layered superlattice material from the soft baked precursor residue.
Other features, objects, and advantages will become apparent to those skilled in the art upon reading the detailed description below in combination with the accompanying drawings.